There is a nice description here:
Basically, parallel imports = gray market. Just sounds nicer, I guess.
Given the quality of fakes now, I think it's hard to distinguish fakes from gray market. Since scammers tend to routinely offer lower levels of criminality as both plausible scenarios and "involving" criminality to deflect complaints, I would tend not to trust something openly proclaimed to be gray market. I would think it would have an excellent chance of being a fake. JMO.